1. Why retirement needs two separate calculations
Most calculators only tell you the corpus you need. But building that corpus is a different problem from spending it sustainably. Retirement planning has two phases:
- Accumulation phase: From today to retirement — you invest, and the corpus compounds.
- Decumulation phase: From retirement to end of life — you withdraw, and the corpus must not run out.
The first phase determines your monthly SIP. The second determines the size of the target. Getting the second wrong (underestimating inflation or lifespan) means the corpus runs out in your 70s — the worst possible outcome.
2. Step 1: Inflate your expenses to retirement
The first calculation is to figure out how much you'll spend in your first year of retirement. That's today's monthly expenses inflated at pre-retirement inflation.
Monthly expense at retirement = Today's expense × (1 + inflation)^years to retire
For a 32-year-old retiring at 60 with ₹75,000/month expenses at 6% inflation:
- Years to retirement: 28
- Inflation factor: 1.06^28 = 5.11×
- Monthly expense at retirement: ₹3,83,000/month (₹46 lakh/year)
Most people are shocked by this number. But it's arithmetic — and ignoring it doesn't make it go away.
3. Step 2: Calculate the corpus needed
The corpus needed is the present value of your future retirement expenses — a growing annuity. The formula uses the "real return" (post-retirement return minus post-retirement inflation):
Real return = (1 + postReturn) ÷ (1 + postInflation) − 1
Corpus = Annual expense × (1 + realReturn) × [1 − (1 + realReturn)^−n] ÷ realReturn
Where n is the number of years in retirement. If real return is negative (inflation exceeds portfolio return), the corpus is larger than a simple multiple of expenses.
💡 The classic "25× rule" (corpus = 25 × annual expenses) assumes a 4% safe withdrawal rate. At current Indian returns and inflation, a more realistic multiple is 28×–35× for a 30-year retirement.
4. Step 3: Project your existing corpus and SIP
Now work out what your current savings will grow to. Two components:
- Existing corpus: Grows at the pre-retirement return for the number of years to retirement.
- Monthly SIP: Each instalment compounds for the remaining months. If you have a step-up SIP, each year's increase must be factored in.
Example: ₹15 lakh existing at 12% for 28 years becomes ₹15L × 1.12^28 = ₹3.2 crore. A ₹20,000/month SIP at 12% for 28 years becomes roughly ₹8.7 crore. Total projected corpus: ₹11.9 crore.
5. Step 4: Compute the gap and the required SIP
If the corpus needed is larger than the projected corpus, you have a gap. The monthly SIP needed to close the gap is:
Required SIP = Gap × monthlyRate ÷ [(1 + monthlyRate)^months − 1]
Where monthlyRate is the monthly pre-retirement return, and months is (years to retirement × 12). This is the amount you need to invest each month — over and above your existing savings and SIP — to hit the target.
6. The four levers when the required SIP is too high
If the required SIP looks unaffordable, you have four levers. Pull them in this order:
- Step up your SIP: A 10% annual step-up starting at ₹20,000 can grow the corpus far more than a flat ₹30,000 SIP. Aligns with income growth.
- Extend the timeline: Retiring at 62 instead of 60 adds 2 years of accumulation and removes 2 years of withdrawal — a double benefit.
- Reduce retirement expenses: A smaller home, a less expensive city, or a simpler lifestyle lowers the target. This is often the most powerful lever.
- Accept a higher post-retirement return: If your health and temperament allow, keeping 40%–50% in equity post-retirement supports a 8%–9% return — but with more volatility.
⚠️ Don't increase your return assumption to "make the numbers work". A 15% pre-retirement return is not realistic for a 28-year horizon. Use 10%–12% for equity-heavy portfolios and be conservative elsewhere.
7. A worked example
A 32-year-old, retiring at 60, with:
- Current expenses: ₹75,000/month
- Existing retirement savings: ₹15 lakh
- Current SIP: ₹20,000/month
- Pre-retirement return: 12%
- Post-retirement return: 7%
- Pre & post inflation: 6%
- Life expectancy: 85 (25 years in retirement)
Calculations:
- Monthly expense at 60: ₹75,000 × 1.06^28 = ₹3,83,000
- Annual expense at 60: ₹46 lakh
- Real return in retirement: 1.07/1.06 − 1 = 0.94%
- Corpus needed at 60: ₹46L × 1.0094 × [1 − 1.0094^−25] / 0.0094 = ₹10.3 crore
- Projected corpus: ₹3.2 Cr (existing) + ₹8.7 Cr (SIP) = ₹11.9 crore
- Surplus: ₹1.6 crore — you're on track
That's a comfortable outcome. But change one assumption — life expectancy to 90 — and the required corpus jumps to ₹12.1 crore, turning a surplus into a small gap.
8. Post-retirement: making the corpus last
Accumulating the corpus is half the job. Making it last 25–35 years is the other half. Three practical principles:
- Withdraw conservatively: 4%–4.5% of the corpus in year 1, adjusted for inflation each year. Higher withdrawal rates risk running out.
- Keep some equity: A 30%–40% equity allocation in retirement supports returns and inflation protection. Pure debt may not outpace inflation.
- Create a bucket structure: 2 years of expenses in cash, 5 years in debt, the rest in equity. Refill buckets in good years, spend from cash in bad years.
✓ A "bucket strategy" prevents you from selling equity in a market crash just to fund living expenses. This single discipline dramatically reduces the risk of running out of money.
9. Common mistakes to avoid
- Using today's expenses as the retirement target: The #1 mistake. Always inflate first.
- Underestimating lifespan: Plan to 85 minimum; 90 is safer if your family history suggests longevity.
- Assuming a high post-retirement return: You can't afford equity-like volatility when you're drawing down. Use 6%–7%.
- Forgetting healthcare: Budget separately for medical costs, or hold a strong health insurance policy into retirement.
- Not reviewing: Retirement planning is not one calculation — it's an annual review. Recalculate every year as income, expenses, and market returns change.
- Starting late: A 30-year-old needs roughly half the monthly SIP of a 40-year-old for the same corpus. Time is the most powerful lever.
- Retiring too early: Every year of early retirement adds roughly 3%–4% to the required corpus.
- Ignoring taxes: Withdrawals from taxable accounts are taxed. Factor in post-tax returns when planning.
10. Final thoughts
Retirement planning is unglamorous but essential. The number is bigger than most people expect — largely because of inflation and a 25-year+ retirement. But the maths is mechanical, and the levers are within your control: how much you save, how you invest, when you retire, and how you plan to spend.
Use this calculator to see your required corpus and monthly SIP. If the numbers look intimidating, don't panic — start where you can, step up every year, and review annually. A plan you actually follow beats a perfect plan you don't.
